Lyme disease is the most common vector-borne disease in the United States. It is caused by the bite of a tick infected with the spirochete, Borrelia burgdorferi, the etiologic agent of the disease. Evaluation of the potential for development of disease from a tick bite is complicated by incomplete knowledge of the diversity of spirochete genotypes in nature and their pathogenic capacities. Furthermore, current laboratory diagnosis rests on a number of serologic tests of varying degrees of sensitivity and reliability, which limits the rapid and specific diagnosis of the disease immediately following a tick bite. During the previous grant period significant progress was made in the molecular typing of B. burgdorferi clinical isolates. In particular, the investigators observed an unequal distribution of genotypes between skin and blood; a particular genotype which is readily cultured from skin was significantly less frequently found in blood. This suggests that the capacity for hematogenous dissemination may vary among different B. burgdorferi genotypes. In addition, progress in the application of a polymerase chain reaction (PCR)-based assay for detection of B. burgdorferi in skin, blood, and synovial fluid was achieved. The investigators propose to extend these findings by pursuing the following specific aims: l) They hypothesize that there is significant genotypic diversity among B. burgdorferi in nature, with the greatest diversity in wildlife hosts and the least in human patients. Molecular typing of B. burgdorferi in ticks and various wildlife hosts will be carried out by PCR-RFLP analysis and DNA sequencing and the number and distribution of genotypes in reservoir hosts, ticks and patients will be compared. 2) Dissemination of different RFLP types will be tested directly in a murine model. I. scapularis colonies infected with single, clonal B. burgdorferi genotypes will be established and potential phenotypic variation will be explored with regard to dissemination, acquisition and transmission. 3) The potential of PCR as a modality for diagnosis of early Lyme disease and Lyme arthritis, particularly in patient blood and synovial fluid specimens, will be further explored. A number of variables will be systematically evaluated with the goal of designing a PCR-based approach which can be effectively employed for reliable diagnosis of Lyme disease. The proposed experiments will provide for a more complete understanding of the infection dynamics of B. burgdorferi. This will contribute to a better appreciation of the risk of contracting Lyme disease from a tick bite. In addition, a reliable PCR-based method for detection of B. burgdorferi in patient blood and synovial should improve accurate diagnosis of early Lyme disease and Lyme arthritis.